In the manufacture of integrated circuits, wafer surface planarity is of extreme importance. Photolithographic processes are typically pushed close to the limit of resolution in order to create maximum circuit density. For 16 megabit dynamic random access memories, minimum critical dimensions, such as word line and bit line width, will initially be in the 0.5.mu.-0.7.mu. range. Since these geometries are photolithographically produced, it is essential that the wafer surface be highly planar so that the electromagnetic radiation used to create a mask may be accurately focused at a single level, thus resulting in precise imaging over the entire surface of the wafer. Were the wafer surface sufficiently non-planar, the resulting structures would be poorly defined, with the circuit being either nonfunctional or, at best, endowed with less than optimum performance.
In order to achieve the degree of planarity required to produce ultra high density integrated circuits, chemical-mechanical polishing or planarization processes are employed. In general, chemical-mechanical polishing (CMP) processes involve holding a semiconductor wafer against a moving polishing surface that is wetted with a chemically reactive, abrasive slurry. Slurries are usually either basic or acidic and generally contain alumina or silica particles. The polishing surface is typically a planar pad made of relatively soft, porous material such as blown polyurethane. The pad is usually mounted on a planar platen.
FIG. 1 depicts a conventional rotational CMP apparatus, generally denoted 10. The apparatus comprises a wafer carrier 11 for holding a semiconductor wafer 12. A soft, resilient pad 13 is typically placed between the wafer carrier 11 and the wafer 12, and the wafer is generally held against the resilient pad by a partial vacuum, friction, or adhesive, etc. Frictional affixation may be accomplished by placing a resilient backing pad of uniform thickness between the carrier and the wafer, the backing pad having a higher co-efficient of friction with respect to the wafer and carrier surface with which it is in contact on opposite sides than the co-efficient of friction of the wafer with respect to the slurry saturated polishing pad. The wafer carrier 11 is designed to be continuously rotated by a drive motor 14. In addition, the wafer carrier 11 is also designed for transverse movement as indicated by the double headed arrow 15. The rotational and transverse movement is intended to reduce variability and material removal rates over the surface of wafer 12. The apparatus further comprises a rotating platen 16 on which is mounted a polishing pad 17. The platen is relatively large in comparison to the wafer 12, so that during the CMP process, the wafer 12 may be moved across the surface of the polishing pad 17 by the wafer carrier 11. A polishing slurry containing chemically-reactive solution, in which are suspended abrasive particles, is deposited through a supply tube 18 onto the surface of polishing pad 17.
FIG. 2 illustrates the basic principles of the conventional rotational CMP process. The polishing pad 17 is rotated at an angular velocity of W.sub.p radians per second (rads./sec.) about axis O. The wafer to be planarized 12 is rotated at an angular velocity of W.sub.w rads./sec., typically in the same rational sense as the pad. It is easily understood that the linear speed (L) of the polishing pad in centimeters/sec., at any given radius (R) in centimeters from axis O, will be equal to W.sub.p r. Experience has demonstrated that the rate of removal of material from the wafer surface is related to the speed with which the pad surface makes contact with the wafer surface.
There are a number of disadvantages associated with the conventional CMP process. For example, most existing CMP systems employ gravity or other means for forcing the wafer against the polishing surface of the pad with an object that a certain amount of slurry remain disposed between the two structures. In conventional CMP, there is no mechanism for addressing the quantity or quality of the slurry disposed between the wafer and polishing pad. Rather, the hope is that a certain portion of the slurry pumped onto the polishing pad will make it between the wafer and pad. U.S. Pat. No. 5,232,875 entitled "Method and Apparatus of Improving Planarity of Chemical-Mechanical Planarization Operations," addresses a portion of this issue. Specifically, this patent documents a chemical-mechanical polishing apparatus wherein slurry is fed from a supply to a network of channels beneath the polishing pad and from there through open pores which extend from a lower surface of the pad to the upper surface of the pad, thereby supplying slurry directly to the wafer-pad interface. Unfortunately, this CMP approach does not address the issue of quality of the slurry at the waferpad interface.
Specifically, debris or film from the surface of the wafer undergoing polishing can be trapped between the wafer and polishing pad, which if unremoved could result in scarring of the wafer surface. Thus, a need still exists in the art for a technique for ensuring/improving the quality of slurry at the interface between a wafer and polishing pad of a chemical-mechanical polishing apparatus. The present invention addresses this need.